The present invention relates to a spread-spectrum packet communication apparatus.
There is currently active research-and-development interest in wireless packet communication systems that employ spread-spectrum technology, more particularly code division multiple access or CDMA technology, to communicate with mobile stations. In typical systems of this type, each packet of data is preceded by transmission of a known reference signal as a header. The receiver uses the header to detect the beginning of the packet, and as a reference signal for coherent demodulation, also referred to as coherent detection. More specifically, the receiver employs the header as a known reference from which to estimate parameters that are then used in phase compensation, or in the weighting of signals received with different delays, thereby overcoming various types of fading. A feature of CDMA systems is that the reference signal can be transmitted with a comparatively high processing gain, to permit more accurate channel estimation, leading to more accurate packet detection and more accurate coherent demodulation.
The term xe2x80x98packetxe2x80x99 will be used below to mean a transmission segment comprising one header followed by payload data. This transmission segment will also be referred to as a transmission frame. A transmission frame must be short enough that channel characteristics do not change significantly over the duration of the frame. Any change that occurs must be small enough to be negligible, or at least small enough to be estimated by simple methods in the receiver. When the receiver is located in a rapidly moving vehicle, such as an automobile traveling at one hundred kilometers per hour, fading is correspondingly fast, and the transmission frame length must be reduced to a value in the range from about half a millisecond (0.5 ms) to about one millisecond (1 ms).
This transmission frame length is much shorter than the data frame length that forms the natural unit of length for error-correcting coding and interleaving. A data frame usually lasts from about ten to about twenty milliseconds (10 ms to 20 ms). Each interleaved data frame must therefore be divided into small packets, and a header must be attached to each packet to create transmission frames with the above length of 0.5 ms to 1 ms.
A problem is that to obtain the benefits of a high processing gain, the header must be at least several symbols long, and sometimes more than ten symbols long. The header alone then occupies a considerable fraction of each packet, typically from about ten percent to about thirty percent, causing an equivalent drop in the rate of data transmission. For communication with slower-moving mobile stations or non-moving stations, this is an unnecessary and undesirable loss of channel efficiency.
It is accordingly an object of the present invention to enable a spread-spectrum communication apparatus to estimate channel characteristics with necessary accuracy while avoiding unnecessary channel overhead.
The invented spread-spectrum communication apparatus has a transmitter that transmits packets of transmit data preceded by respective reference signals to a distant apparatus, and a receiver that receives similar packets of data and reference signals transmitted from the distant apparatus. The receiver uses the received reference signals in demodulating the received packets of data, and generates information indicative of channel conditions between the spread-spectrum communication apparatus and the distant apparatus.
The spread-spectrum communication apparatus also comprises a control unit that estimates the channel conditions from the information generated by the receiver, and adapts the length of the packets of data transmitted by the transmitter according to the channel conditions. The transmitter may transmit configuration information indicating the packet length.
The receiver may use the received reference signals to calculate a channel estimate, which is then used in demodulating the received packets of data. The channel estimate may also be updated during the reception of the packets of data, by an adaptive algorithm, for example. The updating is preferably enabled and disabled according to the channel conditions estimated by the control unit.